Research Findings: The roles of child lexical diversity and maternal sensitivity in the development of young children's inhibitory control were examined in 100 low-income Hispanic Spanish-speaking children. Child communication utterances at age 2½ years were transcribed from 10-min mother-child interactions to quantify lexical diversity. Maternal behavior was rated independently from the interactions. Inhibitory control was measured with a battery of tasks at ages 2½ and 3½. Greater maternal sensitivity was correlated with higher vocabulary at 2½. Greater vocabulary predicted positive growth in child inhibitory control skills from ages 2½ to 3½ in multivariable regression models that controlled for maternal education, family income, the home environment, and mothering quality. Practice or Policy: These findings suggest that supporting vocabulary development in low-income Spanish-speaking children is important for the development of inhibitory control skills, an important foundation for school readiness and academic success.